This invention relates to a method and apparatus for molding such optical glass elements as a glass lens, a prism, etc., for example, and more particularly a method and apparatus for molding glass elements wherein a glass blank is disposed between upper and lower molds, the molds and the glass blank are heated and pressed for press molding the glass blank to obtain an optical glass element.
In the prior art molding apparatus, high frequency induction heating has been used as the heating means of the molds and the glass blank as disclosed in Japanese Laid-Open Patent Publication Nos. 170228/1988 and 45734/1989.
The high frequency induction heating accompanies a problem of penetration depth so that only the surface portion of a portion to be heated made of metal which constitutes a portion of the molds is heated. For this reason, it is difficult to uniformly heat the molds. Especially, where the molds are large, a problem occurs when a high frequency induction coil is disposed around the outer periphery of the molds for heating them.
We have proposed to heat the molds and glass blank by infrared ray lamps as disclosed in Japanese Patent Application No. 407118/1990 to obtain more uniform heating than the high frequency induction heating.
As a result of our various researches regarding the heating of the infrared ray lamps, we have found that either, one of the molds displaces during the press molding operation, or the size of the glass blank of the optical glass element to be molded changes due to the change of the relative position of the molds and the infrared ray lamps. Therefore, it is difficult to obtain an accurate temperature control of the upper and lower molds.
Depending upon the configuration of the optical glass element it is preferable to maintain temperature difference between the upper and lower molds while increasing or decreasing the temperature of those molds or while pressing the molds. Therefore, we have found that it is necessary to accurately control the temperature of the upper and lower molds in order to accurately mold the optical glass element.